The invention concerns a circuit provided with an exposure measuring portion for sensor-controlled distance or range measurement where two linear semiconductor image sensors are provided each having individual sensor elements. Line segments corresponding to the two images and separately obtained from the object are projected onto the image sensors. Switching circuits are connected to the sensor elements which are switchable between two different states and an evaluator is provided for evaluating these two state sensor signals in dependence upon different position displacements with respect to a maximum correlation. Comparing sensor elements are also provided and receive optically generated charge quantities corresponding to sensor elements in the image sensors.
Circuits of this type are specified in German patent application P No. 2,838,647.2, incorporated herein by reference. If they are designed such that the length of a comparing sensor element approximately corresponds to the length of a linear image sensor or exceeds this, then difficulties can result in the determination of an integration time which is required for an exact distance or range measurement. Integration time is here understood as the time span in which an optical charge generation proceeds in the individual sensor element as a result of exposure by means of the projected line segments.
These difficulties occur for example when a small part of the line segment which is projected onto an image sensor and onto the comparing sensor element which adjoins this displays a large amount of brightness, whereas the other, preponderant part of the line segment is dark. In this case, the comparing sensor element forms a mean charge value which is close to the darkness value. The integration time which is determined by the evaluator of the exposure measuring part is then so large that the sensor elements which are directed to the dark points of the line segment have taken up optically generated charges which are only slightly smaller than their reference charge. Therefore, the digital logic states which are set in the associated evaluators are very strongly dependent upon parameter or characteristic variations of the sensor elements and of the evaluator circuits, so that a brightness curve which is in fact not present is simulated along the line segment. If on the other hand the comparing sensor element is designed significantly shorter than the linear image sensors, then in the case of the determination of the integration time, there also results inaccuracies, since the exposure of the sensor elements of the image sensor which are further removed from the comparing sensor elements remain out of consideration.